The broad objective of this project is to understand the conditions which determine whether an antigen-specific interaction between T and B cells results in immunity or tolerance. Work supported by this grant has shown that B cells are tolerogenic antigen presenting cells in vivo. The general strategy for the proposed grant period is to use TCR transgenic T cells to circumvent the low frequency of naive, antigen-specific T cells in normal animals, and so gain direct access to antigen-specific, tolerizing cellular interactions in vivo and in vitro. Antigen- transgenic mice have been made that express a cytochrome c peptide analog covalently attached to the b chain of an MHC class II, I-Ek molecule. APC from these mice are recognized by cytochrome-specific, I-Ek restricted T cells of the AND strain of T cell antigen receptor (TCR) transgenic mice. Resting or activated antigen-transgenic B cells or other APC will be transferred into mice with circulating primed or naive TCR transgenic T cells, and TCR transgenic T cells will be transferred into mice in which antigen is presented only on B cells or on other types of APC. The fate and phenotype of the T cells will be followed by flow cytometry, and their functional capabilities will be studied by secondary challenge with antigen in vitro. Antigen-transgenic mice will be bred to CD40 deficient and B7-1 transgenic mice to study the roles of those molecules in T/B interactions in vivo. The functional consequences for naive T cells of an initial encounter in vitro with antigen on resting normal B cells (which induces T cell proliferation) and resting, CD40-deficient B cells, which do not induce T cell proliferation, but enable T cells to survive and remain responsive to antigen will be compared.